Reference ladder having improved feedback stability

0001This application claims the benefit of U.S. Provisional Application No. 60/388,311, filed on Jun. 14, 2002, which is incorporated by reference herein in its entirety.

BACKGROUND OF THE INVENTION

00021. Field of the Invention

0003The present invention generally relates to a reference ladder having improved feedback stability that can provide reference voltages for an analog-to-digital converters (ADC), and other types of circuits that utilize reference voltages.

00042. Background Art

0005Analog-to-digital converters (ADCs) convert analog signals into a digital format for further efficient processing using digital circuits and/or processors. As digital control and processing are applied to more numerous applications, the demand for ADCs continues to increase. Furthermore, there is an increasing demand for ADCs that are inexpensive and that provide high performance.

0006A conventional ADC often includes a voltage reference ladder (also called a reference ladder) having a resistor ladder with a plurality of taps. Each tap provides a reference voltage that is utilized by the ADC to quantize the incoming analog signal. More specifically, a bank of ADC comparators (or differential input stages) compares the reference voltages from the reference ladder to the analog signal in order to quantize the analog signal and generate a digital output signal.

0007The accuracy of the analog-to-digital conversion is heavily dependent on the accuracy of the reference voltages from the reference ladder. More specifically, the ADC full scale range is controlled by these voltages. Accordingly, a feedback network is utilized to maintain the accuracy of the reference voltages. The feedback network utilizes certain taps of the reference ladder known conventionally as “sense” points or taps, and “force” points (or taps). A voltage or current source is applied at the force taps to control the reference voltages that are produced by the reference ladder. The force taps are usually the first and last taps on the reference ladder. The sense taps are certain taps on the reference ladder that are monitored (or sensed) to check if the desired reference voltages are being produced by the reference ladder. More specifically, an operational amplifier (op amp) compares the voltage across the sense taps with a desired reference voltage (e.g. ADC full scale voltage), and adjusts the voltage or current source at the force taps to produce the desired voltage across the sense taps.

0008A bank of ADC comparators are connected to the taps of the reference ladder and receive the reference voltages. The ADC comparators have associated circuit parasitics that load the sense taps. As a result, the parasitics of the ADC comparators effect the voltage and current at the sense taps, and effectively load the feedback control loop that drives the force taps of the reference ladder. Furthermore, since the sense taps are taken across a subset of the plurality of taps, a portion of the reference ladder resistance is also in the feedback control loop. Both the circuit parasites and the reference ladder resistance contribute to destabilization of the feedback control loop.

0009Accordingly, it is desirable to configure the reference ladder so as to prevent the ADC comparators and the ladder resistance from loading the feedback control loop, so as to improve the loop stability.

BRIEF SUMMARY OF THE INVENTION

0010The present invention is a voltage reference ladder that has improved feedback stability. The voltage reference ladder includes a resistor ladder having a plurality of taps to produce a plurality of output voltages that can be used as reference voltages by an ADC, or another type of circuit. The resistor ladder is driven by a first current source at a first tap of the plurality of taps and by a second current source at a second tap of the plurality of taps. A first feedback network senses a voltage at the first tap and controls the first current source based on the first sensed voltage. A second feedback network senses a voltage at the second tap and controls the second current source based on the second sensed voltage. The first and second taps are at opposite ends of the resistor ladder. Furthermore, the first and second taps each operate as both a force tap and a sense tap of the resistor ladder. This occurs because the first and second taps are connected to the outputs of their respective current sources, and are also sensed by their respective feedback networks.

0011The first feedback network includes a first op amp having a first input connected to the first tap and an output controlling the first current source. The second feedback network includes a second op amp having a first input connected to the second tap and an output controlling the second current source. The reference voltage is applied across a second input of first op amp and a second input of the second op amp, and is determined by scaling-up the ADC full scale range from V_refto V_ref′.

0012A plurality of differential input stages can be connected to corresponding taps of the resistor ladder to receive the plurality of output voltages from the reference ladder. The differential input stages are at least partially isolated from the first feedback network and the second feedback network, thereby improving stability of the first feedback network and the second feedback network when compared to conventional configurations.

0013In another embodiment, the resistor ladder includes a first resistor ladder and a second resistor ladder, each having a plurality of taps. The first resistor ladder is driven by a first current source and a second current source at respective force taps that are located at opposite ends of the first resistor ladder. A second resistor ladder is driven by a third current source and a fourth current source at respective force taps that are located at opposite ends of the second resistor ladder. The first and second resistor ladders are configured to generate substantially identical output voltages across their respective taps.

0014A first feedback network senses a voltage at a first sense tap of the first resistor ladder, and controls both the first current source and the third current source based on the sensed voltage. A second feedback network senses a voltage at a second sense tap of the first resistor ladder, and controls both the second current source and the fourth current source based on the voltage that is sensed at the second sense tap. As such, the first and second feedback networks control the respective current sources of both of the first and second resistor ladders, but only sense voltage from the first resistor ladder.

0015A plurality of differential input stages, corresponding to the taps of second resistor ladder, receive the plurality of voltages from the second resistor ladder. The plurality of differential input stages are at least partially isolated from the feedback networks because the sense taps for the feedback networks are taken from the first resistor ladder, and not the second resistor ladder.

0016Further features and advantages of the present invention, as well as the structure and operation of various embodiments of the present invention, are described in detail below with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is described with reference to the accompanying drawings. In the drawings, like reference numbers indicate identical or functionally similar elements. Additionally, the left-most digit(s) of a reference number identifies the drawing in which the reference number first appears.

<figref idref="DRAWINGS">FIG. 1</figref> illustrates a conventional reference ladder for an analog-to-digital converter.

<figref idref="DRAWINGS">FIG. 2</figref> illustrates a reference ladder according to embodiments of the present invention.

<figref idref="DRAWINGS">FIG. 3</figref> illustrates a reference ladder configuration having a first and second resistor ladders according to embodiments of the present invention.

<figref idref="DRAWINGS">FIG. 4</figref> illustrates external loading of a reference ladder.

DETAILED DESCRIPTION OF THE INVENTION

Conventional ADC

0022The conventional reference ladder <b>100</b> includes a reference generator <b>102</b> that generates a voltage reference V_ref, op amps <b>104</b> and <b>106</b>, and a resistor ladder <b>112</b>. The resistor ladder <b>112</b> includes multiple resistors <b>118</b> that are series-connected, and taps <b>119</b> between the resistors <b>118</b>. The resistor ladder <b>112</b> is driven by a current source <b>108</b> at the top, and by a current source <b>110</b> at the bottom of the resistor ladder <b>112</b>. The taps <b>119</b> of the resistor ladder (between the resistors <b>118</b>) provide the reference voltages for the ADC. In other words, the reference voltages are used by the ADC differential input stages to quantize the analog input. (See <figref idref="DRAWINGS">FIG. 4</figref>)

0023The first and last taps <b>114</b><i>a </i>and <b>114</b><i>b </i>are referred to as force taps, as this is where the current sources <b>108</b> and <b>110</b> drive the resistor ladder <b>112</b>. The taps <b>116</b><i>a </i>and <b>116</b><i>b </i>are referred to as sense taps, as the sense taps are used to sense the voltage on the resistor ladder <b>112</b>.

0024The op amp <b>104</b>, the current source <b>108</b>, and the sense point <b>116</b><i>a </i>form a feedback network <b>120</b>. Likewise, the op amp <b>106</b>, the current source <b>110</b>, and the sense point <b>116</b><i>b </i>form a feedback network <b>122</b>. The op amp <b>104</b> compares the voltage at the sense tap <b>116</b><i>a </i>to the voltage V_refproduced by the voltage generator <b>102</b>, and generates an output based on the difference. The output of the op amp <b>104</b> controls the current source <b>108</b>. Likewise, the op amp <b>106</b> compares the voltage at the sense tap <b>116</b><i>b </i>to the voltage V_refproduced by the voltage generator <b>102</b>, and generates an output that controls the current source <b>110</b>. In other words, the feedback networks <b>120</b> and <b>122</b> drive the current sources <b>108</b> and <b>110</b> to set a desired voltage between the sense taps <b>116</b><i>a </i>and <b>116</b><i>b </i>equal to V_ref.

0025The voltage V_refis set to the “ADC full scale range” so that the feedback networks <b>120</b>, <b>122</b> drive the voltage across the sense taps <b>116</b> to this voltage. The “ADC full scale range” generally represents the voltage that the analog input signal is expected to fall within. Therefore, the taps <b>119</b> within the ADC full scale range should provide a sufficient number of reference voltages to quantize the analog input signal. However, additional reference voltages are required to insure linear operation of the ADC, especially at the edges of the “ADC full scale range”. Furthermore, various ADC architectures, such as folding flash and folding interpolating ADCs, require additional reference voltages that are outside the ADC full scale range. Hence, there are additional resistors <b>118</b> outside the ADC full scale range to generate the additional reference voltages, namely the resistors <b>118</b> between the force points <b>114</b> and the sense points <b>116</b>, which are also in the feedback networks <b>120</b> and <b>122</b>.

0026The ADC full scale reference voltages can be identified as a first set of reference voltages, and the additional references voltages outside the ADC full scale range can be identified as a second set of reference voltages, to produce a the total voltage drop across the resistor ladder <b>112</b>. The first set of reference voltages and the second additional set of reference voltages are delivered to the ADC differential input stages and are generally known as “ADC reference voltages”, shown in FIG. <b>1</b>.

0027The taps <b>119</b> are connected to differential input stages (e.g. comparators) that quantize an analog input signal. For example, <figref idref="DRAWINGS">FIG. 4</figref> illustrates the taps <b>119</b> connected to a bank of differential input stages <b>402</b> that quantize an analog input signal <b>404</b> using the reference voltages from the resistor ladder <b>112</b> of the reference ladder <b>100</b>. The differential input stages <b>402</b> have parasitic s associated with them. These parasitics load the feedback networks <b>120</b> and <b>122</b> via the taps <b>119</b> within the respective feedback networks, causing loop instability. Furthermore, the resistors <b>118</b> in the feedback networks <b>120</b> and <b>122</b> also load the feedback networks and cause instability.

0028<figref idref="DRAWINGS">FIG. 2</figref> illustrates a reference ladder <b>200</b> having improved feedback stability according to embodiments of the present invention. The reference ladder <b>200</b> includes a reference generator <b>202</b> that generates a voltage reference V_ref, op amps <b>104</b> and <b>106</b>, and a resistor ladder <b>204</b>. The resistor ladder <b>204</b> includes multiple resistors <b>118</b> and taps (e.g.<b>119</b>) between the resistors <b>118</b>. The resistor ladder <b>204</b> is driven by the current source <b>108</b> at the top and by a current source <b>110</b> at the bottom of the resistor ladder <b>112</b>. The taps of the resistor ladder (between the resistors <b>118</b>) provide the reference voltages for the ADC. The “reference voltages” produced by the reference ladder <b>200</b> may also be called “output reference voltages” or more generally “output voltages”.

0029The op amp <b>104</b>, the current source <b>108</b>, and the tap <b>206</b><i>a </i>form a feedback network <b>205</b>. Likewise, the op amp <b>106</b>, the current source <b>110</b>, and the tap <b>206</b><i>b </i>form a feedback network <b>208</b>. The op amp <b>104</b> compares the voltage at the tap <b>206</b><i>a </i>to the voltage V_ref′ produced by the voltage generator <b>202</b>, and generates an output based on the difference. The output of the op amp <b>104</b> controls the current source <b>108</b>. Likewise, the op amp <b>106</b> compares the voltage at the tap <b>206</b><i>b </i>to the voltage V_ref′ produced by the voltage generator <b>202</b>, and generates an output that controls the current source <b>110</b>. In other words, the feedback networks <b>205</b> and <b>208</b> drive the current sources <b>108</b> and <b>110</b> to set a desired voltage between the sense taps <b>206</b><i>a </i>and <b>206</b><i>b </i>equal to V_ref′. As will be apparent, the current sources <b>108</b> and <b>110</b> can be configured as voltage sources that produce the necessary current to generate the ADC reference voltages.

0030Based on the discussion above, the reference ladder <b>200</b> is similar to the reference ladder <b>100</b>, except that the first tap <b>206</b><i>a </i>is connected to the input of the op amp <b>104</b>, and the last tap <b>206</b><i>b </i>is connected to the input of the op amp <b>106</b>. In other words, the taps <b>206</b><i>a </i>and <b>206</b><i>b </i>operate as both force taps and sense taps. Stated another way, the force taps and the sense taps are converged to a single pair of taps <b>206</b><i>a </i>and <b>206</b><i>b</i>. Whereas, in the conventional reference ladder <b>100</b>, the force taps <b>114</b><i>a </i>and <b>114</b><i>b </i>are separate and distinct from the sense taps <b>116</b><i>a </i>and <b>116</b><i>b. </i>

0031As a result, the reference feedback networks <b>205</b> and <b>208</b> in the reference ladder <b>200</b> are not loaded by any of the resistors <b>118</b>, or by the differential input stages connected to the taps <b>119</b>. Therefore, the stability of the feedback networks <b>205</b> and <b>208</b> is improved.

0032Another result of converging the force/sense taps <b>206</b><i>a </i>and <b>206</b><i>b </i>is that the ADC full scale voltage is now a fraction of the voltage across the force/sense taps <b>206</b>. Whereas, in the conventional reference ladder <b>100</b>, the ADC full scale voltage is equal to voltage across the sense taps <b>116</b>. Therefore, the voltage V_ref′ produced by the voltage generator <b>202</b> is a scaled-up version of the voltage V_refthat is produce by the voltage generator <b>102</b>, for a given ADC full scale range. More specifically, ratio of V_ref′ to V_refis scaled-up by an amount of: <br /><i>V</i>_ref<i>′/V</i>_ref=[Reference voltages for ADC]/[ADC full scale voltage];<br /> where [Reference voltages for ADC]=the total voltage drop across the resistor ladder. The voltage V_ref′ is scaled-up to accommodate for the voltage drop across the resistors <b>118</b> that are between the force/sense taps <b>206</b> and the ADC full scale range, (namely, the first two resistors and the last two resistors in the voltage ladder <b>204</b>). Whereas, in the reference ladder <b>100</b>, this voltage drop was considered by the feedback network.

0033<figref idref="DRAWINGS">FIG. 3</figref> illustrates another reference ladder <b>300</b> having improved loop stability according to embodiments of the invention. Reference ladder <b>300</b> includes two resistor ladders to separate the ADC op amps connected to the second resistor ladder from the feedback networks connected to the first resistor ladder. More specifically, reference ladder <b>300</b> includes a first resistor ladder <b>310</b> having a plurality of resistors <b>306</b> with multiple taps in between the resistors that includes sense taps <b>308</b><i>a </i>and <b>308</b><i>b </i>and force taps <b>305</b><i>a </i>and <b>305</b><i>b</i>. The sense taps <b>308</b><i>a,b </i>are fed back to the respective op amps <b>104</b> and <b>106</b> for comparison with V_refto form feedback networks <b>302</b> and <b>316</b>. Based on the comparison with V_ref, the output of the op amp <b>104</b> controls a current source <b>304</b>, and op amp <b>106</b> controls a current source <b>314</b>. Similar to the discussion above, the op amps <b>104</b> and <b>106</b> control the current sources <b>304</b> and <b>314</b> so that the voltage across the sense taps <b>308</b><i>a </i>and <b>308</b><i>b </i>is equal to V_ref, which is set to the ADC full scale range.

0034The reference ladder <b>300</b> also includes a second resistor ladder <b>324</b> having a plurality of resistors <b>322</b> with multiple taps in between the resistors <b>322</b> that generate the ADC reference voltages. The second resistor ladder <b>324</b> is driven by current sources <b>318</b> and <b>326</b> at the respective force taps <b>320</b><i>a </i>and <b>320</b><i>b</i>. The second reference ladder does not have any sense taps, as the sensing operation is performed using the first reference ladder.

0035The second voltage ladder <b>324</b> is configured to produce approximately identical voltages at taps between the resistors <b>322</b>, as that produced by the corresponding taps of the first voltage ladder <b>310</b>. To do so, resistors <b>322</b> are selected to have values that are approximately the same as the corresponding resistors <b>306</b> in the first resistor ladder <b>310</b>. Furthermore, the current source <b>318</b> is the same as the current source <b>304</b> and is commonly controlled by the output of the op amp <b>104</b>. The current source <b>326</b> is the same as the current source <b>314</b> and is commonly controlled by the output of the op amp <b>106</b>. Accordingly, the reference voltages produced by the reference ladder <b>324</b> should match those produced by the resistor ladder <b>310</b>, where the voltages from the resistor ladder <b>324</b> drive the differential input stages in the ADC (e.g. differential input stages <b>402</b>).

0036By using two resistor ladders, the differential input stages connected to the taps of the resistor ladder <b>324</b> are isolated from the feedback networks <b>302</b> and <b>316</b>. Therefore, the parasitics associated with the ADC differential input stages do not load the feedback network <b>302</b> or the feedback network <b>316</b>, which improves loop stability of these loops.

0037The reference ladder discussed herein has been illustrated as part of an ADC. This is done for example purposes only, and is not meant to limit the scope of the invention. The reference ladder can be used in other circuit applications besides ADCs, as will be understood by those skilled in the relevant arts.

CONCLUSION

0038Example embodiments of the methods, systems, and components of the present invention have been described herein. As noted elsewhere, these example embodiments have been described for illustrative purposes only, and are not limiting. Other embodiments are possible and are covered by the invention. Such other embodiments will be apparent to persons skilled in the relevant art(s) based on the teachings contained herein. Thus, the breadth and scope of the present invention should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents.